shed its hydrogen. Gupta offers that a
hydrogenated pattern written on graphene could be removed or changed
to switch the state of the underlying
material. This would function in the
same manner as a field-programmable
gate array, he notes.
As with any new material, questions
abound. Take the hydrogen’s bond with
the graphene. A field does not need to
be applied to keep hydrogen on the surface of the graphene, but some research
indicates that hydrogen can move
around on its own atop the material.
The lightest atom on Earth does not
tend to stay in one place for long, Gupta
notes, so its bonding to the graphene
may not be stable. Its constancy at room
temperature “is one of the open questions in the field,” he says.
In addition, several hurdles remain
before hydrogenated graphene can be
used in real-world applications. One
daunting challenge involves graphene’s
honeycomb lattice structure. The lattice’s
carbon atoms are identical, but they fall
into two different classes in terms of
their electronic structure. The hydrogen
must adhere to only one of the two carbon types for magnetization to occur.
Gupta explains that because the
carbon atoms are the same, no thermo-
dynamic preference exists for one atom
over the other. A solution may be to
couple the graphene with something
else. For example, two layers of material
could be oriented in a way that allows
the hydrogen to stick to the preferred
carbon-type sublattice, he suggests.
Another option would be to combine
graphene with a different 2-D material,
such as boron nitride, which also has a
honeycomb lattice structure. If the two
materials were layered, the carbon atoms
in the graphene would differ according
to whether they resided over the boron
or the nitrogen in the layer below.
These layered materials, called hetero-structures, are an active area of research.
Gupta allows that no one has made significant progress yet in growing large-scale structures instead of just at the
micron scale, which is the current capability. Ultimately, layered combinations
of 2-D materials—such as graphene
and boron nitride—may help overcome
many key challenges.
Gupta reports that about 20 other
2-D materials composed of different elements also are the focus of research on
controlling their properties by painting
their surfaces. Using a 2-D material as
Picturing graphene as a canvas, researchers
are painting the honeycomb structure
with hydrogen to confer magnetic and
semiconducting capabilities onto the
single-atom-thick material—as illustrated
in this concept featuring “Mona Lisa.”
a canvas makes it easier for scientists to
explore a material’s properties, he points
out. Unlike with superconductors, which
require scientists to develop new crystal growth methods for each variation,
all that is necessary with a 2-D material is to expose the surface to different
substances. Another approach might be
to place the 2-D material on different
types of substrates. The result of the latter would be a greater range of tunability
with more opportunities for the process,
Another key graphene challenge lies
in preserving its magnetism. The hydrogenated magnetic state is sensitive to the
material’s doping level, Gupta points out.
The typical substrate for graphene is silicon dioxide, the surface of which tends
to feature “puddles” of positive or negative charges. If the puddles absorb the
hydrogen, the magnetic state is killed.
For that reason, the variations in doping
level produced by the substrate also must
“It’s definitely a difficult challenge,”
Gupta says. “I think the proof of princi-
ple is in place, and that is related to hav-
ing the graphene interact with another
Next up for researchers will be refin-
ing graphene-based technologies such as
transistors, which have their limitations.
Gupta explains that transistors must per-
mit switching current on and off with
high ratios, and a graphene transistor
does not turn off very well. However, a
transistor with a hydrogenated graphene
channel may permit increasing the on-
off ratio, he suggests. This could allow
graphene to be used in conventional
Magnetic graphene may allow further
miniaturization of magnetic bits in storage, Gupta offers. Instead of today’s hard
disks having a small dot of magnetic
metal, a layer of graphene patterned with
hydrogenated regions could perform the
same storage tasks. The result would be
much smaller magnetic storage devices.
Another future development could be